Pacemakers provide electrical stimulus to heart tissue to cause the heart to contract and pump blood. Conventionally, pacemakers include a pulse generator that is implanted, typically in a patient's pectoral region just under the skin. One or more leads extend from the pulse generator and into chambers of the heart, most commonly in the right ventricle and the right atrium, although sometimes also into a vein over the left chambers of the heart. An electrode at a far end of the lead provides electrical contact to the heart tissue for delivery of the electrical pulses generated by the pulse generator and delivered to the electrode through the lead.
The conventional use of leads that extend from the pulse generator and into heart chambers has various drawbacks. For example, leads have at their far ends a mechanism, such as tines or a “j-hook,” that causes the lead to be secured to a tissue region where a physician positions the lead. Over time, the heart tissue becomes intertwined with the lead to keep the lead in place. Although this is advantageous in that it ensures the tissue region selected by the physician continues to be the region that is paced even after the patient has left the hospital, it can be problematic if it becomes necessary to move or remove the lead. For example, subsequent to initial implant, it may be determined that an alternate location is preferable for pacing. Similarly, leads can fail. Failed leads cannot always be left in the patient's body, as potential adverse reactions including infection, thrombosis, valve dysfunction, etc., may occur. As such, lead-removal procedures, which can be difficult, sometimes must be employed. The conventional use of leads also limits the number of sites of heart tissue at which electrical energy may be delivered. This is because leads are often positioned within cardiac veins, and multiple leads may block a clinically significant cross-sectional fraction of the vena cava and branching veins leading to the pacemaker implant.
Potential use of leads within the left chambers of the heart may present various difficulties. For example, a thrombus or clot may form on the lead or electrode, and high pumping pressure on the left side of the heart may eject the thrombus or clot into distal arteries feeding critical tissues, which may cause a stroke or other embolic injury. Thus, conventional systems for pacing the left side of the heart instead have threaded a pacing lead through the coronary sinus ostium in the right atrium and through the coronary venous system to a pacing site in a vein over a left heart chamber. While a single lead may occlude a vein over the left heart locally, other veins can sometimes compensate for the occlusion by delivering more blood to the heart. Nevertheless, multiple such leads positioned in veins can cause significant occlusion, particularly in veins such as the coronary sinus when multiple side-by-side leads are used.
There are several heart conditions that may benefit from pacing at multiple sites of heart tissue. One such condition is congestive heart failure (CHF). It has been found that CHF patients have benefited from bi-ventricular pacing—that is, pacing of both the left ventricle and the right ventricle in a timed relationship. Such therapy has been referred to as “resynchronization therapy.” The conventional use of leads limits the number of sites of heart tissue at which electrical energy may be delivered. Similarly, catheters are presently used in the coronary venous system, primarily to pace the left ventricle from the veins. It is known that venous pacing is less efficient at treating CHF than is pacing from the inside wall of the left ventricle.
Wireless Pacing Electrodes (WPEs) have been proposed for the treatment of heart failure through resynchronization of contraction of the right and left ventricles, and for prevention of arrhythmias, including ventricular tachycardia and ventricular fibrillation. A significant issue to be considered in achieving a commercially practicable system is the overall energy efficiency of the implanted system. For example, the energy transfer efficiency of two inductively coupled coils decreases dramatically as the distance between the coils increases. In one example, the WPE contains a battery that is recharged from an antenna located outside the patient. In this implementation, the WPE battery stores only enough energy to pace the heart for a few days, and recharging occurs approximately daily. In another example, a battery-free WPE contains a capacitor with charge-holding capacity sufficient to pace the heart for one or several heartbeats. Energy is transmitted to the WPE from an implanted antenna located outside of the heart, and in patients where multiple WPEs are used, each WPE capacitor is recharged at each heartbeat. Because of the distance between the WPE and the antenna, the coupling between the two may be inefficient, and frequent recharging of an implanted controller that drives the antenna may be required.